This invention relates to apparatus for signalling within a borehole while drilling, and is more particularly concerned with a down-hole signal transmitter for a mud-pulse telemetry system.
Various types of measurements-while-drilling (MWD) systems have been proposed for taking measurements within a borehole while drilling is in progress and for transmitting the measurement data to the surface. However, to date only one type of system has enjoyed commercial success, that is the so-called mud-pulse telemetry system. In that system the mud stream, which passes down the drill string to the drill bit and then back up the annular space between the drill string and the bore wall with the object of lubricating the drill string and carrying away the drilling products, is used to transmit the measurement data from a down-hole measuring instrument to a receiver and data processor at the surface. This is achieved by modulating the mud pressure in the vicinity of the measuring instrument under control of the electrical output signal from the measuring instrument, and sensing the resultant mud-pulses at the surface by means of a pressure transducer.
Current mud-pulse telemetry systems utilize a down-hole signal transmitter which is built into the drill collar. These systems therefore suffer from the disadvantage that, in the event of instrumentation failure in the transmitter, the complete drill string must be withdrawn to enable the faulty part to be replaced. Moreover the combined transmitter/drill collar is very costly to produce.
One such system comprises a turbine which is driven by the mud flow and drives an electrical generator for supplying the measuring instrument with power. The turbine also drives a hydraulic pump for displacing a throttling member to produce the required mud pulses. The displacement of the throttling member is determined by the electrical output of the measuring instrument. However, it is of the utmost importance that the mud should not penetrate into the housing containing the electrical generator and associated mechanism, and accordingly a rotating seal surrounds the shaft coupling the turbine to the generator. Such a seal is difficult to manufacture and prone to failure leading to the complete drill string having to be withdrawn and the drill collar having to be replaced.
It is an object of the invention to provide a generally improved down-hole signal transmitter for a mud-pulse telemetry system.